Short Answer:
The longbow was very difficult to use requiring a lot of experience and strength. But the crossbow was very easy to use and anybody could use it.
Long Answer:
The bow is basically a big spring made of wood.

When it bends back energy is stored in the two limbs. Let go this strain and that energy is released in shooting the arrow in more than a 160 ft/second.

Left hand was used to hold the bow and right hand was used to pull the string and release the arrow.

Just about every culture in the world developed a bow. Shorter ones for sitting on horseback, longer ones for distance and power. And one of the most famous and effective weapon were the English longbow.
13th century, English Longbow:
- Length 6 ft
- Maximum effective range 300 yards.
They were very popular in the 1300s. In fact this was the terror weapon of the 1300s and early 1400s.
Back then the English and the French were at a war for about a hundred years. That's why they call it the hundred years war. The English had the advantage of the longbow superiority which came from the material used to create it.
Longbows were cut from one piece of wood. They made good longbows. Its outer wide wood was soft and could withstand a lot of tension. The inner wide wood could resist compression. Combined they gave longbow a deadly force that the French knights learnt to fear.

But the English also had the bow of a particularly nasty arrowhead made of hardened steel whose only purpose was to penetrate armour and kill.

But long bows were much more difficult to use and required lots of practice.
Although the draw weight of a typical English longbow is disputed, it was at least 360 newtons (81 pounds-force) and possibly more than 600 N (130 lbf), with some estimates as high as 900 N (200 lbf). Considerable practice was required to produce the swift and effective combat shooting required. Skeletons of longbow archers are recognisably adapted, with enlarged left arms and often osteophytes on left wrists, left shoulders and right fingers.
It was the difficulty in using the longbow that led various monarchs of England to issue instructions encouraging their ownership and practice.
During the reign of Edward III of England, laws were passed allowing fletchers and bowyers to be impressed into the army, also forbidding men and boys to play football or golf and encourajing them to practice archery instead.
Finding men powerful enough to shoot a long bow was a tough call.
And that's when another type of bow came.
11th century, medieval crossbow:
- Maximum effective range 60 yards.
The earliest record of the crossbow was in China around 500 B.c. But they probably have been used even earlier.
The Greeks and the Romans had them too. But by the time they entered many more European countries they were in demand more than ever.
The cross bow is a small bow mounted on a duo wooden stand or tiller with a trigger mechanism to release the bow.

Because the bow was short and took a lot of energy to pull back the arms, it needed both hands to hook it into the knob and all that energy is stored up in the bows arms. Release the trigger and the bow shot out like a bat out of Hell.

Though the longbow required a lot of practice and muscle the crossbow was a piece out of a cake. Anybody could load these. They were quite similar to pulling the trigger of a gun.
The feet were used to hold the crossbow against ground and both the hands to load it pulling the arrow with the back. Then one simply had to aim and pull the trigger to fire.
Advantage of longbow -
- Experienced archers could shoot 20 arrows in a minute. They also travelled a longer distance.
Advantage of crossbow -
As the hundred years of war went on the number of English longbow archers fell. The French began to gain the upper-hand because they had more crossbows. The longbow had had its day.
Though longbow was the sniper rifle of the day it could not compete with the popularity of the crossbow, which some special forces use even today.
Sources:
- https://www.youtube.com/watch?v=qsAUKRbaZ9E (primary)
- https://en.wikipedia.org/wiki/Longbow
- https://en.wikipedia.org/wiki/English_longbow
- https://www.historylearningsite.co.uk/medieval-england/the-longbow/
- http://www.thebeckoning.com/medieval/crossbow/cross_l_v_c.html
It might be hard to convince anyone to develop a program to develop those skills for 7 years or whatever it takes in an army that doesn't have it at all. And as Ben Crowell says, the whole of England were required by law to train bow. It might be hard to justify bothering with that much effort when you can invest in other forms of attack
– Timo Mar 02 '17 at 16:55